Interposers are used for integrated circuit packaging, typically for space transformation, which is for routing connections between semiconductor dies and packaging components. For example, semiconductor dies may have tightly-spaced bonding pads, which are inconvenient for being packaged onto package substrates. Interposers may thus be used to increase the pitches of the semiconductor dies. In this case, an interposer has a side with a first pitch, which corresponds to the pitches of the respective semiconductor dies bonded thereon. Bonding pads on the other side have a second pitch greater than the first pitch, and the second side may be bonded onto a package substrate.
FIG. 1 illustrates a conventional package including interposer 10, and die 12 bonded onto interposer 10. Usually, interposer 10 includes substrate 11, which is typically formed of organic materials or ceramics. Metal connections 14 are formed in dielectric layers 16. Through the routing in dielectric layers 16, metal connections 14 transform a greater pitch of solder bumps 18 into a smaller pitch of solder bumps 20. Die 12 may further include through-silicon vias (TSV) 22 for making an electrical connection from solder bumps 20 to the opposite side of die 12. Die 12 is flip-chip bonded onto interposer through solder bumps 20.
With the increasing down-scaling of integrated circuits, it is preferred that the pitch of bumps 20 is as small as the pitch of bonding pads on semiconductor die 12. Existing formation technology has scaled the pitch of TSVs 22 to about 20 μm. However, a bottleneck exists for downscaling the pitch of solder bumps 20. Currently, for bonds made through solder bumps, the minimum achievable pitch is only about 130 μm, which is significantly greater than the pitch of TSVs 22. As a result, the capability of through-silicon via technology is not fully utilized. A new bonding technique is thus needed.